1. Field of the Invention
This invention relates to a method for coating the surface of inactive granules closely with a layer of fine metal oxide powder several nanometers in thickness. More particularly, this invention relates to a method for coating the surface of inactive granules by causing inactive granules having water adsorbed in advance on the surface thereof to contact a metal alkoxide thereby hydrolyzing the metal alkoxide and consequently covering the granules with the hydrolysate.
The method of this invention finds utility in such applications as production of a sintering auxiliary.
2. Prior Art Statement
As methods heretofore commonly employed for the production of granules coated with a layer of fine metal oxide powder several nanometers in thickness, there can be mentioned a method which comprises mutual adsorption between particles of a microfine powder approximately not more than 10 nm in diameter and granules approximately 1 .mu.m in diameter through the medium of a surfactant by adjusting the delta (.delta.) potential and controlling the pH value [S. Hirano, Design of Complex Material II Organic Complex Material, (Progress of Chemical Industry 23 Molecule Chemical Industry Chapter 7) edited by Chemical Engineering Society], a method which comprises thoroughly mixing a microfine powder and granules in a liquid and spray drying the resultant mixture [Y. Kawashima Journal of Color Material Society, Vol. 55(9), p 657 (1982), and a method which comprises subjecting a microfine powder and granules to mutual friction in dry air thereby attaining mutual adhesion therebetween by virtue of static electric power [F. Yokoyama et al, Abstracts of Papers Read at Granule Formulation and Design Symposium p 67 (1986)]. In case of these conventional methods, the success of the deposition of a microfine powder on granules is at the mercy of probability and the control of layer thickness requires considerable knowhow. In the products of these methods, the coating layer is a coarse film formed by mere cohesion of the particles of microfine powder and the ratio of microfine powder which escapes cohesion is large. The methods, further, have a disadvantage that they are not capable of easily producing a submicron order coat of high economic value.
In the circumstances, it is necessary to develop a method capable of coating inactive granules with a dense layer of microfine powder without entailing any of the drawbacks mentioned above.
This invention has been produced to meet this need. An object of this invention is to provide a method for coating the surface of inactive granules with a dense layer of microfine powder and a convenient method for allowing free control of the thickness of the layer to be formed as described above.